This invention relates to power converters. More particularly, the invention relates to a DC-to-DC power converter operating in the flyback switch mode.
A self-oscillating self-referencing flyback switch mode power converter commonly incorporates a power transformer wound on a ferrite core. The power transformer has a primary winding in which energy is stored during an energy storage cycle. Current from a high voltage DC power source is passed through the primary winding during the energy store pulse. The energy store cycle or pulse is eventually terminated, and the stored energy is delivered through the power transformer to a set of secondary windings which develop secondary power supply voltages to a load. Termination of the energy store cycle occurs with the opening of the circuit for the primary current. The collapsing magnetic field in the primary winding represents the stored energy transferred to the secondary, and is commonly referred to as the energy transfer cycle.
The more advanced switching regulator power supplies utilizing the flyback switch mode include a servo feedback control loop from the secondary to the primary current circuit to obtain regulation control of the secondary voltages. At least one of the prior-art flyback switch mode power converters also incorporate primary regulation control of the secondary voltages. Primary regulation control is obtained by detecting the level of the primary current flowing in the primary winding during the energy store cycles. The detected primary current produces a control voltage used in cooperation with the servo feedback control signal from the secondary to determine when to terminate each the energy store cycle.
An example of a flyback switch mode power converter incorporating both primary and secondary regulation control for the secondary voltages is disclosed in U.S. Pat. No. 4,225,913. A current sensing resistor is connected between the high voltage DC power source and the high side terminal of the primary winding. A current switch is provided in the circuit from the low side terminal of the primary winding for coupling this terminal to the ground return for the high voltage DC power source. Operating through an opto-coupler to isolate the secondary circuits from the primary is a servo feedback control voltage developed from one of the secondary voltages. The feedback servo voltage acts in cooperation with a primary current level sensing signal developed from the voltage across a current sensing resistor to provide the control to a current switch for terminating each energy store cycle.
The power converter disclosed in U.S. Pat. No. 4,225,913 has the disadvantage that sensing of the primary current is at high voltage potentials. These high voltages must be transformed down to low voltages in order to provide switching control signals to the current switch which is in the low voltage side of the primary winding. To perform the required level translation from high-to-low voltages, a stress transistor is required. The presence of this stress transistor has the further disadvantage that additional circuitry is required with higher likelihood for converter failure, as well as the introduction of distortion of the converter switching signals by the presence of the transistor in the circuit.
Therefore, it would be advantageous to provide a DC-to-DC power converter having both primary and secondary regulation control in which the primary current sensing occurs at low voltage potentials to provide the primary regulation control of the flyback current switch, without the need for level translation from high voltages to low voltages.